Although the ideal diode of the present invention may be used in any application requiring a diode, the invention is particularly beneficial in circuits that rectify relatively high voltages and currents, such as in a power application for rectifying an AC voltage.
FIG. 1 illustrates a simple application of a diode 10, whether it is a typical pn junction diode or an ideal diode. An AC source 12 is assumed to apply a 1 kHz sinusoidal voltage, having peaks of +100V and −100V, to the anode of the diode 10. The AC voltage is rectified by the diode 10 and somewhat smoothed by a capacitor 16 for application of the resulting voltage to a load 18. The load 18 draws current from the diode 10 when the diode is conducting and draws current from the capacitor 16 when the diode 10 is reverse biased. Therefore, there is some voltage droop when the diode 10 is reverse biased.
FIG. 2 illustrates a single cycle of operation of the circuit of FIG. 1. Both the sinusoidal anode voltage 20 and the smoothed voltage 22 applied to the load 18 are shown. During steady state operation, the capacitor 16 charges to the peak +100V when the diode 10 is forward biased, and the voltage droops to about +40V when the diode 10 is reverse biased. The droop amount depends on the capacitor 16 and the load 18. Therefore, the diode 10 becomes forward biased (i.e., conducting) when the input AC voltage exceeds +40V and becomes reversed biased (i.e., off) after the AC voltage reaches its peak of +100V.
Many applications require the conversion of AC power to DC power, such as shown in FIG. 1. This can be done in many ways, but the most basic implementations use one or more rectifying diodes, such as in a full-wave bridge circuit. Independent of the AC frequency or topology (half-wave, full-wave, single-phase, or multi-phase), a diode-based rectifier will always include a voltage drop equal to the forward voltage of the diodes at the operating load current. Associated with this voltage drop is a power dissipation equal to the voltage drop multiplied by the diode current.
An ideal diode circuit automatically switches a low-loss switch, such as a MOSFET, when it is detected that the anode terminal is at a voltage higher than the cathode terminal. Comparators and other circuitry are used in the control circuit. The control circuit may be powered by the voltage applied to the ideal diode. Typically, the control circuit is referenced to ground, so the components in the control circuit must be able to operate with the full anode-to-ground voltage. When the voltage to be rectified is a relatively high voltage (e.g., 120 VAC), the control circuitry needs to be fabricated using special techniques to handle the high voltage without breakdown. This increases the size and cost of the control circuitry.
NMOS transistors are frequently used as the low-loss switch since their conductivity is typically better than that of PMOS transistors. Since the source of the NMOS transistor is coupled to the anode terminal, the control circuitry requires a voltage-boosting charge pump, referenced to earth ground, to generate a gate voltage higher than the anode voltage. The charge pump generally runs continuously to maintain the boosted voltage needed to drive the NMOS transistor. This continuous running of the charge pump consumes power. This boosted voltage also requires the control circuitry to handle a voltage higher than the anode voltage without breakdown.
It is also known to eliminate the earth ground reference from the boost circuit, but the boost circuit can only run when the ideal diode is off. Therefore, the ideal diode must be periodically forced off to generate the boost voltage.
What is needed is an ideal diode circuit that uses low-voltage control circuitry (e.g., up to 20V), even when the voltage to be rectified may be a high voltage (e.g., greater than 120 VAC). What is also needed is an ideal diode circuit that can generate a boosted voltage for controlling an NMOS transistor without interfering with the normal ideal diode operation. Such an ideal diode circuit can then be operated accurately in AC applications and with high voltages to be rectified.